1. Field of the Invention
This invention pertains to electronic filters that utilize piezoelectric resonators. More particularly this invention pertains to microwave piezoelectric filters that utilize piezoelectric resonators fabricated as part of a monolithic device.
2. Description of the Prior Art
In the prior art, microwave electronic filters have been manufactured that utilize piezoelectric resonators that are fabricated in monolithic form on a substrate. See e.g. "Development of Miniature Filters for Wireless Applications", Lakin, Kline, McCarron, IEEE Trans. Microwave Theory and Techniques, Vol. 43, No. 12, December 1995, pp. 2933-2929; "Thin Film Bulk Acoustic Wave Filters for GPS", K. M. Lakin, G. R. Kline, and K. T. McCarron, 1992, Ultrasonics Symposium Proc. pp. 471-476; High-Q Microwave Acoustic Resonators and Filters," by Lakin, Kline and McCarron, IEEE Trans. on Microwave Theory and Techniques, Vol. 41, No. 12, December 1993, p. 2139. One such method of fabricating piezoelectric resonators consists of first depositing a layer of conducting material upon the upper surface of a non-conducting substrate and then removing portions of the conductor by etching so as to leave a desired pattern of conductors. A layer of piezoelectric material is then deposited upon the upper surface followed by the deposition of another layer of conducting material. Portions of the upper-most conducting material are then removed by etching leaving a second desired pattern of conductors on the upper surface of the piezoelectric material. Each portion of the layer of piezoelectric matter that is sandwiched between two conductors, together with the bonding conductors may then act as a piezoelectric resonator. In some prior art devices, these resonators are supported upon one or more layers of material that provide, in effect, either a fixed surface having a high mechanical impedance to vibration, or a "free" surface having a low mechanical impedance to vibration. See, e.g. U.S. Pat. Nos. 3,414,832 and 5,373,268. In some prior art devices, areas of the substrate located beneath the resonators are removed so as to leave the resonators as thin membranes. Another method for fabricating resonators is the classical method wherein conducting electrodes are fabricated upon the lower and upper surfaces of a piezoelectric crystal plate or slab.
FIG. 1A depicts the layout of a filter of the prior art in which a plurality of resonators are fabricated upon a wafer or substrate 1 by using a sequence of deposition and etching processes in which conductors and piezoelectric material are deposited upon, and removed from the surface of a substrate so as to form a layer of piezoelectric material sandwiched between upper and lower patterns of conductors. In the layout depicted in FIG. 1A, the boundaries of conductors located upon the upper surface are drawn with solid lines and the boundaries of conductors located beneath the layer of piezoelectric or other material are drawn with dashed lines. In addition, the area covered by ground conductor 2, which is located on the upper surface 3 of the device and which conductor acts as an electrical ground, is indicated by shading.
Each resonator consists of a thin layer of piezoelectric material sandwiched between two layers of conducting material, with the volume of the piezoelectric material that comprises each resonator being defined approximately by the overlapping areas of the two conductors. For example, resonator X11 consists of a layer of piezoelectric material sandwiched between and approximately bounded by the area of overlap of conductor 4, which conductor 4 is located on the surface of the substrate, by conductor 5, which conductor 5 is located beneath the layer of piezoelectric material. The resonator includes the portions of the upper and lower conductors that bound the piezoelectric layer and which approximately define the boundaries of the resonator. The two areas over which conductor 5 overlaps ground conductor 2, approximately define the boundaries of resonators X12A and X12B. Similarly, the area of overlap of conductor 5 by conductor 6 approximately defines the boundary of resonator X13A. In a similar manner, the locations of resonators X13B, X14A, X14B, X15A, X15B, X16A, X16B and X17 are approximately defined by the areas of overlap of the conductors on the upper and lower surfaces of the piezoelectric material. The filter has an input terminal 7 and an output terminal 8 relative to ground conductor 2.
FIG. 1B is a schematic drawing of a simplified equivalent circuit of the filter depicted in FIG. 1A. FIG. 1C is a simplification of the schematic diagram of FIG. 1B in which the pairs of resonators connected in series, that is having a serial connection between them, e.g. X13A and X13B are represented by single, equivalent resonators, e.g. X13, and the pairs of resonators connected in parallel, e.g. X12A and X12B, are represented by single, equivalent resonators, e.g. X12. As indicated in FIG. 1C, the equivalent circuit of the filter is, in effect a combination of series and shunt resonators connected in the form of a ladder. One side of the ladder is the signal or "hot" side of the ladder, and the other side of the ladder is the electrical ground. The "hot" side of the ladder consists of resonators connected in series to each other (the "series" resonators) and the rungs of the ladder consist of "shunt" resonators connected between the ground side of the ladder and the connections between the series resonators.
In this specification, and in the claims, the words "series resonator(s)" are used to describe one or more resonators that either by themselves, or in combination with one or more other resonators together operate as a "series resonator" in the sense that the "hot side", or signal carrying side, of the filter circuit passes through these resonators. Similarly, the words "shunt resonator(s) are used to describe one or more resonators that either by themselves, or in combination with one or more other resonators together operate as a "shunt resonator" in the sense that at least one electrical terminal of the combination of shunt resonators is connected to the ground side of the filter. Resonators may be connected to each other in a serial combination, sometimes referred to as a series connection, as being connected in series with each other or "in series". The resonators also may be connected to each other in a parallel combination, sometimes referred to as a parallel connection or "in parallel".
The input 7 and output 8 of the filter, as well as the all of the resonators are symmetrically located, and symmetrically connected, relative to ground conductor 2. As a consequence the path length for a ground current flowing in ground conductor 2 from the portion of ground conductor 2 adjacent to input 7 to the ground connection of X12A is the same as the path length to X12B. As a consequence the inductances associated with these paths are also equal. The paths for the currents flowing in the ground conductor and the associated inductances similarly are equal for resonators X14A and X14B, etc. The inductances in the ground paths cause a small shift in the resonant frequencies of the respective shunt resonators.
When more resonators are added to a filter that is arranged as depicted in FIG. 1A, the ratio of the length to the width of the filter becomes greater, i.e. has a higher aspect ratio, which high ratio may give rise to undesirable electrical and/or physical properties.
FIG. 2A depicts a filter of the prior art that is similar to the ladder filter depicted in FIG. 1A, that includes two additional series resonators and two additional shunt resonators. The electrical path along the "hot" side of the ladder filter in FIG. 2A, i.e. along series resonators X21 to X23A to X23B to X25A to X25B X27A to X27B to X29 has been changed to a meandering path in order to shorten the length of the filter making the filter more compact and reducing its aspect ratio. The device depicted in FIG. 2A, includes shunt resonators X22A, X22B, X24A, X24B, X26A, X26B X28A and X28B. FIG. 2B is a simplified schematic diagram of the filter depicted in FIG. 2A showing the series and shunt resonators.
Unfortunately, the asymmetrical layout in FIG. 2 of the components relative to the ground conductor provides path lengths within the ground conductor from the vicinity of input and output connectors to the connections between the shunt resonators and ground which are no longer equal. As a consequence the "ground path" inductances associated with these paths, also, are no longer equal. The unequal ground path inductances shift the resonant frequencies of the respective shunt resonators unequally, which unequal shifts degrade filter performance. In particular, the out-of-band rejection by the filter is significantly degraded.